GROUND MAGNETIC SURVEYING
- o Ground Magnetometer Surveying Overview (these are all hot linked to the headings below)
- o Ground Vs Airborne Magnetometer Surveys
- o Ground Surveys
- o Magnetic Survey Instrumentation
GROUND MAGNETICS OVERVIEW
Ground magnetometer surveys measure the amplitude of the earth’s magnetic field at discrete points along regularly spaced lines over a target area. The technique exploits variations in the distribution and properties of magnetic minerals in the lithosphere to map a range of geological attributes including lithology, structure and alteration as well as both regolith and sedimentary processes.
Magnetometer surveys are extensively used in mineral exploration, coal mining, archaeology and environmental surveys.
Mineral exploration is a major commercial application where it is used in the location of world-class mineral deposits either by direct detection or by locating structurally prepared areas (e.g. shear zones or faults) with coincident alteration (magnetite destruction). The technique is becoming increasingly important in the coal mining industry where it may be used in exploration, resource estimation and mine planning. Ground magnetometer surveying is important in quarrying and gemstone exploration with applications that include mapping ‘blue metal’ for concrete aggregate and road base and the detection of alluvial concentrations of gold and precious gemstones.
Countries such as Australia, Canada and the USA invest in systematic airborne magnetic surveys of their respective land masses to map geology and thereby facilitate the discovery of new mineral deposits or the location of basin structures prospective for economic deposits of coal or oil and gas reservoirs. These surveys are typically conducted at a flying height above ground of 80-100m with a line spacing of 400m. They may be followed up at Exploration Licence scale with more detailed, higher resolution helimag or low level fixed wing surveys at a flying height of 25-50m (terrain permitting) and a line spacing of around 25-50m.
GROUND VS AIRBORNE MAGNETOMETER SURVEYS
Where an exploration target is relatively shallow (less than say, 200m), or it is expected to be enveloped by a magnetite depleted or magnetized alteration zone, initial airborne survey work may be followed up by high resolution ground magnetometer surveying with a line spacing of 10 to 50m and a station spacing of 1m to map alteration, structure and lithology. Following processing and interpretation, the data may be used to plan the location, depth and orientation of drill holes.
Comparison of the resolution of low level helimag (lower portion of the image) and ground acquired magnetometer data (upper portion of the image). The helicopter acquired data lacked the resolution to map alteration associated with the targeted gold ore bodies
Depending on the nature of a target magnetised body, the intensity of the magnetic field surrounding it declines at a rate that is either inverse distance squared or inverse distance cubed. As such, the height of a magnetometers sensor will have a significant impact on the resolution of the magnetic data.
This figure compares the resolution of ground magnetometer and low level helimag data acquired with the same line spacing of 25m. The sensor height for the ground data is 1.8m and for the helimag data is 15m..
Acquisition of high resolution GPS located magnetic data – New England Fold Belt, QLD, Using Geometrics G859 caessium Vapour Magnetometer
GROUND SURVEY APPLICATIONS
The principle applications of ground magnetometers surveys may be summarized as:-
- o Enhancement of geological mapping.
- o Mapping lithology
- o Mapping alteration systems
- o Mapping magnetic mineralization
- o Mapping structure and structural relationships
o Input to geotechnical planning of mine design and operations.
Coal Exploration & Development
o Detailed mapping of unknown igneous features such as dykes, sills, flows and plugs that sterilize resources and significantly impact on long wall mining operations.
- o Detailed mapping of basement faulting and fracture systems.
- o Enhancement of existing geological mapping.
- o Mapping paleochannels and other hydrological features.
- o Detecting areas of burnt coal (ignition by lightning strike) and siderite contamination.
- o Input to development of preliminary resource estimation through constrained 3D modelling.
- o Input to mine design through resource definition.
- o input to geotechnical assessment
- o Locate buried petrol tanks, concrete, drums etc
- o Locate old mine workings
- o Locate dykes, sills, flows etc that may impact on road and railway planning and design.
- o Input to mine design – detection of geohazards.
MAGNETICS SURVEY INSTRUMENTATION
We use fast sampling high resolution Scintrex Envi CS magnetometers with inbuilt GPS guidance and Geometrics G858 Caesium Vapour magnetometers coupled with a Micronics MAGuider DGPS/GPS Engine with “Bluetooth”, a Novatel WAAS /omniSTAR antennae (no external cables), a “Bluetooth enabled 2007S PDA with guidance software and degaussed lightweight backpacks. Our base magnetometers are Geometrics G856AX and Scintrex Proton Precession Base Stations.
High resolution ground acquired Total Magnetic Intensity Data surveyed by IMTgeophysics. Line spacing is 20M with a 1M data interval. The survey covers a significant portion of a producing goldfield in Central QLD. Magnetics was successfully used to target mineralised alterations zones with numerous new ore bodies being located and successful mined. Areas of potassic alteration and silicification are evident within the image as well as siginificant structural and lithological data. The western two-thirds of the image is mostly undulating soil covered grazing country while the eastern area is hilly moderate timbered country. Data is used with permission of the client.